p38 MAPK as an essential regulator of dorsal-ventral axis specification and skeletogenesis during sea urchin development: a re-evaluation

A Survey on Tubulin and Arginine Methyltransferase Families Sheds Light on P. lividus Embryo as Model System for Antiproliferative Drug Development

Tubulins and microtubules (MTs) represent targets for taxane-based chemotherapy. To date, several lines of evidence suggest that effectiveness of compounds binding tubulin often relies on different post-translational modifications on tubulins. Among them, methylation was recently associated to drug resistance mechanisms impairing taxanes binding. The sea urchin is recognized as a research model in several fields including fertilization, embryo development and toxicology. To date, some α- and β-tubulin genes have been identified in P. lividus, while no data are available in echinoderms for arginine methyl transferases (PRMT). To evaluate the exploiting of the sea urchin embryo in the field of antiproliferative drug development, we carried out a survey of the expressed α- and β-tubulin gene sets, together with a comprehensive analysis of the PRMT gene family and of the methylable arginine residues in P. lividus tubulins. Because of their specificities, the sea urchin embryo may represent an interesting tool for dissecting mechanisms of tubulin targeting drug action. Therefore, results herein reported provide evidences supporting the P. lividus embryo as animal system for testing antiproliferative drugs.

Expression of exogenous mRNAs to study gene function in echinoderm embryos

With the completion of the genome sequencing projects, a new challenge for developmental biologists is to assign a function to the thousands of genes identified. Expression of exogenous mRNAs is a powerful, versatile and rapid technique that can be used to study gene function during development of the sea urchin. This chapter describes how this technique can be used to analyze gene function in echinoderm embryos, how it can be combined with cell transplantation to perform mosaic analysis and how it can be applied to identify downstream targets genes of transcription factors and signaling pathways. We describe specific examples of the use of overexpression of mRNA to analyze gene function, mention the benefits and current limitations of the technique and emphasize the importance of using different controls to assess the specificity of the effects observed. Finally, this chapter details the different steps, vectors and protocols for in vitro production of mRNA and phenotypic analysis.

MAPK and GSK3/ß-TRCP-mediated degradation of the maternal Ets domain transcriptional repressor Yan/Tel controls the spatial expression of nodal in the sea urchin embryo

In the sea urchin embryo, specification of the dorsal-ventral axis critically relies on the spatially restricted expression of nodal in the presumptive ventral ectoderm. The ventral restriction of nodal expression requires the activity of the maternal TGF-β ligand Panda but the mechanism by which Panda restricts nodal expression is unknown. Similarly, what initiates expression of nodal in the ectoderm and what are the mechanisms that link patterning along the primary and secondary axes is not well understood. We report that in Paracentrotus lividus, the activity of the maternally expressed ETS-domain transcription factor Yan/Tel is essential for the spatial restriction of nodal. Inhibiting translation of maternal yan/tel mRNA disrupted dorsal-ventral patterning in all germ layers by causing a massive ectopic expression of nodal starting from cleavage stages, mimicking the phenotype caused by inactivation of the maternal Nodal antagonist Panda. We show that like in the fly or in vertebrates, the activity of sea urchin Yan/Tel is regulated by phosphorylation by MAP kinases. However, unlike in the fly or in vertebrates, phosphorylation by GSK3 plays a central role in the regulation Yan/Tel stability in the sea urchin. We show that GSK3 phosphorylates Yan/Tel in vitro at two different sites including a β-TRCP ubiquitin ligase degradation motif and a C-terminal Ser/Thr rich cluster and that phosphorylation of Yan/Tel by GSK3 triggers its degradation by a β-TRCP/proteasome pathway. Finally, we show that, Yan is epistatic to Panda and that the activity of Yan/Tel is required downstream of Panda to restrict nodal expression. Our results identify Yan/Tel as a central regulator of the spatial expression of nodal in Paracentrotus lividus and uncover a key interaction between the gene regulatory networks responsible for patterning the embryo along the dorsal-ventral and animal-vegetal axes.

Specification of the embryonic axes is an essential step during early development of metazoa. In the sea urchin embryo, specification of the dorsal-ventral axis critically relies on the spatial restriction of the expression of the TGF-ß family member Nodal in ventral cells, a process that requires the activity of the maternal determinant Panda. How the spatially restricted expression of nodal is established downstream of Panda is not well understood. We have discovered that, in the Mediterranean sea urchin Paracentrotus lividus, the spatial restriction of nodal on the ventral side of the embryo requires the inhibitory activity of a transcriptional repressor named Yan/Tel. This finding suggests a molecular mechanism for the control of nodal expression by the release of a repression. We found that this release requires the activity of two families of kinases that we identified as the MAP kinases and GSK3, a kinase which, intriguingly, was previously known as a key regulator of patterning along the animal-vegetal axis. We discovered that phosphorylation by MAPK and GSK3 triggers degradation of Yan/Tel by a β-TRCP proteasome pathway. Finally, we find that Yan/Tel likely acts downstream of Panda in the hierarchy of genes required for nodal restriction. Our study therefore identifies Yan/Tel as a new essential regulator of nodal expression downstream of Panda and identifies a novel key interaction between the gene regulatory networks responsible for patterning along the primary and secondary axis of polarity.